The influence of terpenes on host selection and mass aggregation of Ips grandicollis was demonstrated by combining host- and insect-produced chemical attractants. Field populations exhibited a greater response to combinations of loblolly pine terpenes and extracts of male beetle frass than to either attractant tested separately. The attractiveness of frass extract to female and, to a lesser extent, male beetles was enhanced by the addition of d,l-camphene, d-limonene, and methyl chavicol. Myrcene in combination with frass extract attracted more female beetles than either substance tested separately. In laboratory bioassays, combinations of frass extract and host terpenes were less attractive than terpenes tested alone, and more attractive than frass extract tested alone. The discrepancies between the field and laboratory data illustrate that conclusions may be misleading if made from studies based primarily on laboratory results. The addition of synthetic population-aggregating pheromones frontalin and trans-verbenol, from Dendroctonus bark beetles, to loblolly pine terpenes depressed the attractiveness of the terpenes and rendered them less attractive to Ips grandicollis.